I believe that the number of thrust reversers has a lesser effect on landing roll distance as it is increased. For a simplified model of a 4-engined aircraft under poor runway conditions (ice/snow) the reduction in landing roll length for 2 thrust reversers is approx.30-40% but increasing the number of thrust reversers from 2 to 4 results in only a further 10% reduction. If weight or other factors are critical then getting rid of two of your thrust reversers does not present such a big landing performance degrade. I believe the VC-10 was originally designed with 4 reversers but then dropped to two after it was found that in service the stopping performance of two was satisfactory.
As previously stated by another poster i think the most likely reason for selecting to keep the reversers on the inboard engines is to minimise lateral moments should one reverser fail.